Method of joining aluminum and ferrous members



April 8, 1969 w. H. FRISKE ET AL 3,436,805

METHOD OF JOINING ALUMINUM AND FERROUS MEMBERS Filed Aug. 9, 1965 FIE. FIE. E

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United States Patent C 3,436,805 METHOD OF JOINING ALUWUM AND FERROUS MEMBERS Warren H. Friske, Canoga Park, and Edward C. Supan,

Northridge, Califl, assignors to North American Rockwell Corporation, a corporation of Delaware Filed Aug. 9, 1965, Ser. No. 478,155 Int. Cl. B231; 31/02, 35/24 US. Cl. 29-482 13 Claims ABSTRACT OF THE DISCLOSURE Our invention relates to a method of joining aluminum and ferrous members, and more particularly to a method of making metallurgically sound transition joints between tubular components of aluminum and ferrous base alloys.

Joints between aluminum and ferrous members are required for many applications in apparatus and process systems where the structural and metallurgical properties of both metals are required. The differences between aluminum and steel in strength, fabrica-bility, weight, corrosion resistance, and thermal expansion, require that both metals be used together in one system, each for its own characteristics. For example, in a nuclear reactor the process tubes enclosing the fuel elements may be of aluminum or of an aluminum alloy because of the low thermal neutron absorption cross section of aluminum, whereas headers, grid plates, or other structural members to which the process tubes are joined, will be of steel be cause of its desirable strength, fabricability, and lower cost. There are also applications of aluminum-steel components in cryogenic systems. High quality joints between aluminum and ferrous metals, metallurgically sound and leak-tight, are therefore necessary for many current applications.

Conventional brazing and welding methods, however, have not been successful in meeting the strict standards for such joints. Among the basic problems associated with the fabrication of such joints are large stresses at the joint interface due to differences in thermal expansion of the metals, and formation of :brittle intermetallic compounds of iron and aluminum by diffusion across the joint interface which may cause failure in service. There is therefore need for an improved bonding process for joining ferrous and aluminum members. As used in the present application and claims, the term aluminum includes aluminum metal and its alloys with either metallic or ceramic components such as the commercially available aluminum alloys consisting of about 4-12 wt. percent aluminum oxide dispersed in an aluminum matrix. The term ferrous metal is intended to designate metals and alloys at least 50% of which are iron, and includes the various types of stainless steel.

Accordingly, the principal object of our present in- 3,435,865 Patented Apr. 8, 1969 vention is to provide an improved method of joining ferrous and aluminum members.

Another object is to provide such a joint which possesses high strength and gas leak-tight integrity.

Another object is to provide an improved method of joining aluminum and ferrous metals which does not require the use of welding or brazing techniques.

Another object is to provide a method of joining tubular aluminum and ferrous members.

Still another object is to provide a method of joining aluminum and ferrous metals for high temperature applications wherein the formation of brittle, intermetallic aluminum and iron compounds is avoided.

A further object is to provide such a method which can be performed relatively quickly and economically and is suitable as a production method.

Other objects and advantages of the present invention will become apparent to those skilled in the art from the following description and appended claims.

In the drawings, FIG. 1 is a cross section representation of transition joint components,

FIG. 2 is a cross section of a completed transition joint, and

FIG. 3 is a cross section of an apparatus for fabrication of tubular transition joints.

In accordance with our present invention, aluminum and ferrous members may be joined by providing a tapered edge on the ferrous member and then contacting it with the aluminum member under pressure at elevated temperature. This results in an effective, leak-tight joint by back-extrusion of the aluminum over the tapered surface of the ferrous member. The deformation of the aluminum surface by the tapered ferrous member at bonding pressures exposes a clean aluminum surface at the joint interface free of surface contamination, which significantly contributes to the formation of the high quality bond. The surface of the contacting edge of the ferrous member is tapered to a cross section width less than that of the cross section of the associated joining surface of aluminum and the taper preferably extends to a knife-edge. There are several advantages of the tapered edge; the aluminum surface is penetrated easier which exposes the fresh aluminum surface, and aluminum flow along the tapered edge facilitates union of steel and aluminum.

For further details concerning the present invention reference is made to FIG. 1 which shows the steel 2 and aluminum 4 transition joint components prior to joining, steel member 2 having a double knife-edge 6 with about a 60-75 angle to the edge from the horizontal; knifeedges with other angles may be appropriately used. In FIG. 2 is seen a completed transition joint with backextrusion of the aluminum over the ferrous member.

For higher temperature applications of the present aluminum-ferrous joints (for example, above about 650 F.), or for such lower temperature applications Where it is desired to avoid the formation of intermetallic aluminumiron compounds during hot pressing, we find that sucht compound formation may be avoided by the use of a diffusion barrier. The interleaf material may be applied to the ferrous or aluminum member (preferably to the knife-edge of the steel) by conventional techniques such as electroplating, vacuum or vapor deposition, and pack cementation; a preplaced foil may also be used. Satisfactory interleaf barrier materials are tantalum, tungsten, molybdenum, niobium, chromium, gold, silver, and nickel, or any combination of two or more thereof. Of these, the preferred diffusion barrier materials are niobium, tantalum, and chromium. Thin layers of the diffusion material are employed, for example, in the range of about 0.2-3 mils. The diffusion material functions by preventing contact between the aluminum and steel to avoid formation of iron-aluminum intermetallics; further, there is no eutectic or other compound formation between the barrier material and the aluminum or iron, with the exceptions noted below. There is also no evidence of degrading interdiffu sion after prolonged testing at elevated temperatures. The interleaf material conforms to the shape of the knifeedge and is accordingly deformed during the joining step which is characterized by the back-extrusion of aluminum.

The aluminum and ferrous surfaces are joined by hot pressing at elevated temperatures using conventional equipment. The joint components are prepared for bonding by degreasing the surfaces, liquid honing the steel knife-edge, and abrading the aluminum joint area by such means as abrasive paper or a Wire brush. The parts are then rinsed in acetone, alcohol, or the like, and the diffusion barrier material, when employed, is applied on the steel or an interleaf foil placed between the joining surfaces. The joint members are placed in a fixture which maintains conformity of the surfaces, and pressure applied at temperature.

The temperature, pressure, and time parameters may satisfactorily vary over a considerable range and are interrelated in that higher pressures or temperatures may compensate for shorter times and vice versa. The particular gonditions are dependent to some measure upon the properties of the alumium and ferrous members. For example, aluminum metal or a low strength aluminum alloy may be bonded to steel at a lower temperature and/or pressure than for higher strength aluminum alloys. As a general case, it is found that the temperature may satisfactorily be about 750-1200 F., under loads of at least about 20,000 psi of cross section of the aluminum member in its non-tapered section, for periods of time of about 3-30 minutes. Good quality joints may be obtained at higher loads, above about 35,000 p.s.i., although no advantage is gained thereby.

The preferred temperature for hot pressing is dependent upon the strength of the aluminum, and is one which gives good aluminum flow characteristics during back-extrusion over the tapered ferrous member. For example, the aluminum alloy XAPOOl is bonded in the range of about 900-1150 F. Such other aluminum alloys as Type 6063 may be bonded as low as about 800 F. The upper bonding temperature is, in some cases, limited by the particular diffusion barrier selected, and should be below that which gives a eutectic between aluminum and the barrier material. The only ones of the present barrier materials which form eutectics in the above temperature range are gold (Al-Au, 977 F.) and silver (Al-Ag, 1051" F.) and with such metals bonding is performed below such temperatures.

The resulting joined member may then be directly taken to ambient conditions, for example by removing the members from under load into ambient air. However, while no special cooling regime is necessary after the bond is formed, it is preferred to slowly cool the joint under load to a temperature no higher than of at least about 600 F. to minimize any cracking due to stresses at the bond interface resulting from differential thermal expansion of the components. The joint is then removed from the fixture and permitted to air cool to ambient temperature. For example, the joint may be cooled at a rate of about F. per minute to 300 F., and preferably at a rate of about 5 F. per minute to room temperature. The hot pressing is conducted in a manner to avoid oxidation of the joint interface. This ordinarily requires that the joining be conducted in an inert atmosphere such as is provided by vacuum or an inert gas such as argon. However, if the joint interface is assembled in such a manner as to prevent oxidation of the ferrous member, the joining might be performed in air.

An apparatus for fabrication of tubular transition joints .4 between ferrous and aluminum tubular members is shown in FIG. 3. The ferrous and aluminum joint components 2 and 4 are placed in the hot pressing apparatus 8, and are radially restrained by an outer die 10 and an inner mandrel 12 positioned on a die base 14. The die set is fabricated from hardened high-speed tool steel (AISI Type T1 or the like). The assembly is heated by an induction coil 16 surrounding the die, and bonding pressure is applied by a hydraulic press 18 acting vertically on stainless steel tube 2. Thermo-couples 20 are positioned in die 10 and mandrel 12 to provide temperature control during hot pressing. This apparatus is positioned in an atmosphere enclosure (not shown) which provides the controlled atmosphere. In one embodiment, such retort is flushed with argon and evacuated by a mechanical vacuum pump prior to the hot pressing operation.

The following examples are offered to illustrate the present invention in greater detail.

Example I An aluminum alloy tube containing approximately 6% A1 0 dispersed in an aluminum matrix (sold under the trade name XAPOOI by The Aluminum Company of America) was extruded, having the dimensions 1.93 in. CD. by 0.117 in. wall thickness, 3 in. length. A similar size Type 304 stainless steel tube had double knifeedges machined at one end of the tube at an angle of 60 from the horizontal. The joint components were prepared for bonding by degreasing, liquid honing the stainless steel knife-edge, and abrading the aluminum joint area, following which the parts were rinsed in alcohol. A 0.3 mil coating of chromium was electroplated onto the stainless steel knife-edge. The tubular components were than placed in the hot pressing apparatus shown in FIG. 3 and were bonded together under vacuum at a temperature of 1000 F. under a load of 30,000 p.s.i. of aluminum tube cross section for 10 minutes. The joint was cooled under load to about 200 F. in the retort at a rate of about 5 F. per minute, and then removed from the retort and allowed to cool to room temperature.

Testing and evaluation of the aluminum alloy-stainless steel transistion joints were based upon determination of helium leak-tight integrity, long-term isothermal heat treatments, thermal cycling in various media, pressure testing, tensile testing, and metallographic examination of joint cross sections.

Isothermal heat treatments were conducted in an air or vacuum atmosphere at a temperature of about 650 F.-950 F. for about 1000-2500 hours in order to determine the extent and effect of any diffusion interaction at the joint interface. Thermal cycling tests were made using various heating and cooling media. For example, joints were subjected to a plurality of cycles (e.g. 10- 1000) of heating in air to 750 F. and quenching in water at room temperature or cooling to room temperature in air. One joint was subjected to 13 air-water cycles without leaking, and other joints remained helium leaktight after cycles from 750 F. to room temperature.

After the isothermal heat treatment and thermal cycling tests, helium leak tests were made. The joints were generally helium leak-tight, and such leakage as was observed was very small, of the order of 10' cc. (STP)/sec. of helium. Pressure tests employed argon as the pressurizing medium. Test temperatures ranged from room temperature to 780 F. and pressure at failure ranged from 825 to 1425 p.s.i.g. at estimated hoop stresses of 6700 to 26,- 800 p.s.i. Tensile strength tests were conducted at room temperature, 650 F., and 750 F. on longitudinal segments of transition joints. Ultimate tensile strengths of specimens tested at room temperature range at 20-30 k.s.i., and for specimens tested at 650 F. and 750 F. values ranged from about 5 to 12 k.s.i. Metallographic examinations of the joint cross section in the as-bonded conditions after various thermal treatments showed intimacy of contact at the bond interface between the stainless steel and the aluminum alloy. There was no formation of intermetallic compounds.

In conclusion, testing of the joints showed that a high quality, strong, leak-tight bond was established between the stainless steel and aluminum which withstood thermal cycling, isothermal heat treatments for extended periods, and the efiects of high gas pressures. The suitability of such stainless steel-aluminum joints for service under severe environmental conditions was thus established.

Example II The procedure of Example I is followed except that no interleaf material is employed and the joint is bonded at a temperature of 950 F. and a pressure of 31,500 p.s.i. aluminum tube cross section for minutes. The joint is subjected to the testing described in Example I, and a bond of similar high quality is determined to have been obtained.

Example III The procedure of Example I is followed except that a 1 mil foil of tantalum is used as the interleaf material and the joint is bonded at a temperature of 1050 F. and a pressure of 30,000 p.s.i. aluminum tube cross section for 10 minutes.

Example IV The procedure of Example I is followed except that aluminum metal is used, the stainless steel has a 30 knife-edge and a 2 mil niobium foil is used as the interleaf material. The joint is bonded at a temperature of 1000 F. at a pressure of 30,000 p.s.i. aluminum tube cross section for 10 minutes.

The foregoing examples are illustrative rather than restrictive of the present invention which should be understood to be limited only as indicated in the appended claims.

Having thus described the present invention, we claim:

1. A method of joining an aluminum member and a ferrous member at corresponding edges which comprises:

(a) providing a tapered edge on the ferrous member,

(b) contacting said tapered edge of the ferrous member with a corresponding flat edge of the aluminum member, and

(c) heating said contacting edges of said members at an elevated temperature below the fusion temperature of said members while applying sufficient pressure thereto for said tapered edge to penetrate said fiat edge and while excluding air from the joint interface,

(d) said temperature and pressure being suflicient to cause flow of the aluminum over said tapered edge to form a high-strength substantially leak-tight bond between said members at their contacting edges.

2. A method of joining an aluminum member and a ferrous member at corresponding edges, which comprises: '(a) providing a tapered edge on the ferrous member, (b) interposing a thin diffusion barrier material between said tapered edge of the ferrous member and a corresponding flat edge of the aluminum member, (c) said barrier material being selected from at least one metal of the class consisting of tantalum, niobium, chromium, gold, silver, nickel, tungsten and molybdenum, and (d) contacting said ferrous and aluminum members with the interposed barrier material at said corresponding edges under sufficient pressure for said interposed barrier material and said tapered edge to penetrate said flat edge and at an elevated temperature below the fusion temperature of said members and below the temperature of eutectic formation of aluminum and said barrier material, (e) said temperature and pressure being suflicient to cause flow of said aluminum over said tapered edge to form a high-strength, leak-tight bond between said members at their contacting edges.

3. The method of claim 1 wherein said tapered edge is tapered to a knife-edge.

4. The method of claim 1 wherein the bonding step is performed at a temperature of about 750 -1200" F.

5. The method of claim 1 wherein the bonding step is performed at a pressure of at least about 20,000 p.s.i.

6. The method of claim 1 where the bonding step is performed for a period of about 3-30 minutes.

7. The method of claim 1 wherein the bonding step is performed at a temperature at about 750-1200 F., under a load at about 20,000-3S,000 p.s.i., for a period of about 3-30 minutes in an inert atmosphere.

8. The method of claim 2 wherein said diffusion barrier is selected from the class consisting of tantalum, niobium and chromium.

9. The method of claim 2 wherein said tapered edge is tapered to a knife-edge having an angle of about 60-75" from the horizontal.

10. The method of claim 2 wherein said ferrous member is stainless steel and said aluminum member is an alloy consisting essentially of about 4-8 wt. percent A1 0 distributed in an aluminum matrix, and said members are bonded together at a temperature of about 900- 1150 F. under a load of about 25,000-35,00 0 p.s.i. for a period of about 3-30 minutes in an inert environment.

11. The method of claim 10 wherein the resulting joint is slowly cooled under said load to a temperature no higher than about 600 F., whereafter the load is removed and the joint permitted to cool to ambient conditions without further temperature and pressure adjustments.

12. A method of forming a joint between tubular members of steel and aluminum at corresponding ends, which comprises:

(a) providing a knife edge on one end of the steel tube,

(b) interposing a 0.2-3 mil thick diffusion barrier metal between said knife edge end of the steel tube and a corresponding flat end of the aluminum tube,

(c) said barrier metal being selected from the class consisting of tantalum, niobium, chromium, gold, silver, nickel, tungsten and molybdenum,

(d) contacting said ferrous and aluminum members with the interposed barrier metal at said corresponding ends of said members at a temperature of about 750-1200 F. but below the temperature of eutectic formation of aluminum and said barrier metal,

(e) under a load of about 25,000-35,000 p.s.i. for a period of about 3-30 minutes in an inert environment so that there is a flow of aluminum over said knife edge, and

(f) cooling the resulting joint to form a high-strength substantially leak-tight bond between said tubular members at their contacting ends.

13. A method of forming a joint between corresponding ends of tubular members of stainless steel and an aluminum alloy consisting essentially of about 4-8% A1 0 dispersed in an aluminum matrix, which comprises:

(a) machining a knife edge on one end of said steel tube,

(b) electroplating a thin coating of chromium onto said knife edge,

(0) contacting the plated knife edge end of said steel tube with a corresponding flat end of the aluminum tube,

((1) heating said contacting ends of said tubular members in vacuum at a temperature of about 1000" F. under a load of about 30,000 p.s.i. for about 10 minutes so that there is a flow of aluminum over said knife edge,

(e) cooling the resulting joint under load to a temperature of at least about 300 F. at a rate of about 5-10 F. per minute, and then (f) removing the load and permitting the joint to cool 8 to ambient temperature without further temperature 2,809,422 10/ 1957 Schultz 29-4975 and pressure adjustments to form a high-strength 2,908,073 10/ 1959 Dulin 29-498X substantially leak-tight bond between said tubular 2,917,818 12/1959 Thomson 29-1962 members at their contacting ends. 3,292,256 12/ 1966 Morgan 29501 X References Cited 5 JOHN F. CAMPBELL, Primary Examiner.

UNITED STATES PATENTS R. F. DROPKIN, Assistant Examiner.

2,763,058 9/1956 McCullough 29497.5 X 3,367,020 2/1968 Watson 1 29 47s 2,s3s,397 12/1950 Duch 29-501X 10 29196.2, 196.6, 197, 494, 497.5, 498, 504 

